| An important consequence of reservoir creation is the production and bioaccumulation of neurotoxic methylmercury (MeHg) through the food web into fish. The FLooded Upland Dynamics EXperiment (FLUDEX) at the Experimental Lakes Area in NW Ontario was designed to test the hypothesis that MeHg production in reservoirs is related to the amount, and subsequent decomposition, of flooded organic matter. From 1999–2001, three upland forests that varied in the amounts of organic carbon stored (OC) in vegetation and soils (Low C, 30 900 kg C ha−1; Medium C, 34 900 kg C ha−1 ; and High C, 45 900 kg C ha−1) were flooded from spring to autumn with low OC, low-MeHg water pumped from a near-by lake. Within the framework of the FLUDEX, this thesis examined whole-reservoir rates of methylation or demethylation based on net MeHg exports from reservoirs and MeHg pools in soils, periphyton, zooplankton, and fish. There was an initial pulse of MeHg production in all upland reservoirs in the first two years post flood (120–1590 ng m−2 day −1), followed by net demethylation (360–1230 ng McHg degraded m−2 day−1) that reduced the pools of MeHg in the reservoirs. The reservoir with the highest amount of stored OC produced the most MeHg. Large increases in MeHg stores in soils (231–3230 mg ha−1) compared to those in water (32–131 mg ha−1) and biota (0.1–50 mg ha−1) indicated that flooded soils were the main sites of methylation. Reservoirs were always net sources of MeHg to downstream environments (0.01–1.24 mg ha−1 day−1).; This whole-ecosystem scale research was complemented with two smaller studies designed to examine the process of Hg methylation in reservoirs. Enclosure and three-year litterbag experiments simultaneously quantified production of MeHg and decomposition by-products following inundation of 12 plant tissues commonly found in boreal regions. MeHg production was greater in less easily decomposable tissues than in more labile tissues, contradictory to the hypothesis that the production of MeHg is primarily related to rates of OC decomposition. This study should assist hydroelectric utilities in making informed decisions about selecting sites for future reservoir development to reduce MeHg contamination of reservoir fisheries. |
